The present invention relates to transistors, and more particularly to reducing a leakage current and a parasitic capacitance in bipolar transistors.
In bipolar transistors there often exists a relationship between the base size and the device performance. Consider, for example, the transistor in which the base overlies the collector and the emitter overlies the base. In some such transistors made for high current (and, therefore, high power) applications, base contact regions are provided on two opposite sides of the emitter to reduce base resistance and allow larger drive currents. Hence, the base is made sufficiently large to allow a sufficient overlap between the base and the base contact regions and a sufficient spacing between the base contact regions and the emitter.
In some transistors made for low power applications, only one base contact region is provided. On the side of the emitter on which no base contact region is provided the base can be made smaller. Making the base smaller leads to a smaller collector-base capacitance which in turn increases the transistor speed.
Making the base smaller is particularly advantageous in low power applications since the low power requirement tends to reduce the transistor speed. However, making the base smaller can lead to a larger collector-emitter leakage current (current ICEO defined as the collector-emitter current with open base). Indeed, suppose that the base is rectangular in plan view and is surrounded by a field insulator such as silicon dioxide. On the side of the emitter on which no base contact region is provided the corners of the rectangular base are brought closer to the emitter to make the base smaller. As is known, the proximity of the base corners to the emitter may lead to a large collector-emitter leakage current.
Another problem with bipolar transistors is a collector-base leakage current, that is, the base recombination current that flows into the collector. There is thus a need for a bipolar transistor which has lower collector-base and collector-emitter leakage currents and a low collector-base capacitance and which is suitable for high speed low power applications.